In order to transmit high-frequency signals such as clock signals, the differential signal transmission is utilized, in which two signals of different polarities are transmitted.
FIG. 7 is a circuit diagram showing an example of the conventional differential signal transmitting apparatus. The differential signal transmitting apparatus has a transmitting-side board 10 and a receiving-side board 11. The transmitting-side board 10 has a transmitting circuit 1, a transmission line 4a (positive side), and a transmission line 4b (negative side). The receiving-side board 11 has a receiving circuit 2, a transmission line 5a (positive side), a transmission line 5b (negative side), a matching-termination circuit 6, a transmitting-side board connecting terminal 7a (positive side), and a transmitting-side board connecting terminal 7b (negative side). The transmitting circuit 1 and the receiving circuit 2 are each constituted by, for example, a large scale integrated (LSI) circuit.
While the transmitting-side board 10 remains connected to the receiving-side board 11 via the transmitting-side board connecting terminals 7a and 7b, the positive signal included in the differential signal transmitted from the transmitting circuit 1 is input to the positive side of the receiving circuit 2 via the transmission line 4a, transmitting-side board connecting terminal 7a, transmission line 5a and the positive side of the matching-termination circuit 6. Similarly, the negative signal included in the differential signal transmitted from the transmitting circuit 1 is input to the negative side of the receiving circuit 2 via the transmission line 4b, transmitting-side board connecting terminal 7b, transmission line 5b and the negative side of the matching-termination circuit 6.
As shown in this figure, in the differential signal transmitting circuit, the transmitting circuit 1 and the receiving circuit 2 are mounted on different boards, and transmission lines 5a and 5b and the matching-termination circuit 6 are mounted on the receiving-side board 11. In the differential signal transmitting circuit, the positive and negative sides of the receiving circuit 2 are set to the same potential, rendering the differential potential of the circuit 2 undefined, if the transmitting circuit 1 is disconnected from the receiving circuit 2 because of the expansion slots. In this case, a through current may flow in the receiving circuit 2 or an undefined logic state may propagate into the receiving circuit 2. Consequently, a logic problem may arise.
The undefined logic can be avoided if the receiving circuit is a single-end receiving circuit merely by connecting a pull-up resistor of high resistance or a pull-down resistor of high resistance to the input. However, the undefined logic cannot be avoided in the differential signal transmitting circuit if the matching-termination circuit 6 is provided on the differential signal receiving side.
As a conventional technique related to the present invention, a circuit is available, in which a selector connects a pull-up resistor and a pull-down resistor to the positive and negative sides of the differential input of the receiving circuit when the transmitting circuit is found not connected, and disconnects these resistors from the differential input when the transmitting circuit is found not connected. (See, for example, Patent Document 1.)
Patent Document 1: Jpn. Pat Appln. Laid-Open Publication No. 2001-169314